1. Field of the Invention
This invention relates to an electron beam generator for employment in the fabrication of semiconductor devices and other devices. More particularly, this invention relates to such an electron beam generator which employs double deflection of the electron beam such that the beam strikes the target normal to the surface thereof.
2. Description of the Prior Art
The active elements in an integrated circuit chip are formed by a series of processes each of which, according to the prior art, require a masking step whereupon a pattern is formed on a photo-resist material by optical methods. The active elements thus formed generally are not any smaller than 2 microns in dimension and it is believed that the resolution that can be obtained by such optical lithography processes has reached its limit.
Electron beam lithography methods have been employed to achieve higher resolution but have not been greatly successful because of the time involved to generate the patterns due to the low intensity of the electron beam which requires long exposure time for the electron beam resist material employed. Another disadvantage of prior art electron beam generators is that the focusing lens was placed relatively close to the target which allowed for faster scanning of the target but only over a relatively small area. Thus, the scan cycle had to be repeated many times in order to cover a single circuit chip and even many more times to cover the entire target area.
A high current electron beam generator is disclosed in the Wolfe et al U.S. Pat. No. 3,814,975 which employs a cathode needle formed of a single crystal tungsten which needle is coated with zirconium. Other methods of applying zirconium are disclosed in the Charbonnier et al U.S. Pat. No. 3,374,386. Such an electron beam source can generate an electron beam having a current of a thousand amperes per square centimeter. With such a high current electron beam generator, the focusing mechanism can be adapted to provide a large image focal distance relative to the object focal distance such that minor angular deflection of the beam will result in relatively large scans of the target area and yet fully expose the electron beam resist materials without requiring a very slow scan.
However, even with small angular deflections, distortions appear in the pattern being generated at the target surface due to the unevenness of the target surface. Although the semiconductor wafer may be relatively flat before the process of forming the respective integrated circuits therein, the wafer will go through a number of heat cycles for the different steps required in the formation of such circuitry with a resultant warping of the surface. The resultant pattern distortions become more critical as the image spot size of the beam and the required tolerances become smaller. It is, therefore, desirable to produce a scanning electron beam which strikes the target normal to the target surface in order to minimize distortions of the generated pattern.
While the present invention is primarily adapted for employment in the fabrication of semiconductor devices, it can also be employed in the formation of other devices such as magnetic bubble memories, and any place where an electron beam lithography might be required. In addition, the present invention might be employed to form optical masks for later employment in such fabrication or may be employed to form patterns directly on the semiconductor or magnetic wafers.
It is then an object of the present invention to provide an improved electron beam generator.
It is another object of the present invention to provide an improved electron beam generator for employment in electron beam lithography.
It is still another object of the present invention to provide an improved electron beam generator which will generate an electron beam that will strike the target area normal to the surface thereof.